The present invention relates to a modified polytetrafluoroethylene fine powder having good mechanical properties, in particular, pressure resistance, and a method for the production of the same.
Polytetrafluoroethylene (PTFE) for molding roughly includes two types of powders, that is, a powder which is obtained by finely pulverizing granular resins prepared by suspension polymerization (granular resin), and a powder obtained by coagulating polymers from latexes prepared by aqueous dispersion (emulsion) polymerization and drying the coagulated polymers (fine powder), and both powders are practically used. These two types of the powders are processed by entirely different molding methods. For example, the former powder is processed by compression molding or ram extrusion molding, while the latter powder is processed by paste extrusion molding which is carried out by compounding liquid lubricants in the powder, or calender molding. Such PTFE is supposed to have a very high molecular weight, and cannot substantially be melt-processed. Thus, the above-described special molding methods are employed.
Powders of low molecular weight PTFE are called xe2x80x9cwaxesxe2x80x9d, and distinguished from the above PTFE for molding. Such low molecular weight PTFE powders are often used to modify the properties of other thermoplastic resins, thermosetting resins, coatings, inks, oils, etc. by blending, by making use of the excellent properties of PTFE, for example, lubricity. In connection with the properties of the low molecular weight PTFE, it is known that such PTFE has flowability in a melt state, and its molded articles are brittle and do not have sufficient mechanical strength for the practical applications.
As explained above, a PTFE fine powder is mainly shaped by a paste extrusion method in which a liquid lubricant is compounded in the PTFE fine powder and the compound is extruded with an extruder in the form of a rod or a tube. The extruded articles can be used as such, or rolled to form a sheet, and used as a sealing material in a non-sintered state. Alternatively, they are sintered and used as molded articles such as tubes, wire-coatings, etc.
Since PTFE fine powders are supplied to such special molding methods and applications, they are required to have high productivity in the paste extrusion molding process, and rolling and expansion processability. In addition, they are required to have sinterability and dimensional stability during sintering. Sintered molded articles are required to have good mechanical properties and transparency. It is known that all the required properties are largely governed by the properties of the PTFE fine powders used as raw materials.
However, such required properties cover a wide variety of properties from the shaping properties of the PTFE fine powders to the properties of the final molded articles. In general, conventional PTFE fine powders, which have been developed, may cost some properties to improve other properties. For example, some PTFE fine powders have good moldability, but the molded articles made of such PTFE fine powders have low mechanical properties. When the PTFE fine powders are evaluated from the viewpoint of moldability, some have good paste extrusion moldability, but low rolling properties or expandability.
Thus, many proposals have been made to improve the properties of PTFE fine powders.
For example, a method for improving PTFE while maintaining its inherent non-melt processability is known, which method comprises copolymerizing about 1 wt. % or less of other fluorine-containing monomer as a modifier with tetrafluoroethylene (TFE). The copolymer obtained by such a method is named modified PTFE and differentiated from melt-processable TFE copolymers (see JP-B-37-4643, JP-B-50-38159 (=U.S. Pat. No. 3,819,594) and JP-B-56-26242).
JP-B-37-4643 and JP-B-56-26242 disclose a method for improving paste extrudability at a high reduction ratio (HRR. RR (reduction ratio)=a ratio of the cross section of the cylinder of an extruder for paste extrusion into which a paste is charged to the cross section of the outlet of an extrusion die). However, the disclosed PTFE fine powder cannot be used in applications which require heat resistance at high temperature, since it provides molded articles having low heat stability.
JP-B-50-38159 discloses the production of fluoroalkyl vinyl ether-modified PTFE fine powder having a low standard specific gravity and a low melt viscosity, and describes that such modification can improve the mechanical properties, in particular, flexural life of the polymer.
JP-A-64-1711 discloses fluoroalkyl vinyl ether-modified PTFE fine powder having a low standard specific gravity and a high melt viscosity, and describes that the rollability and expandability of the polymer can be improved.
Furthermore, JP-A-7-165828 (=U.S. Pat. No. 5,641,571) discloses a PTFE micropowder comprising TFE and about 3 wt. % or less of a copolymerizable monomer, which has a polydispersibility of 1.5 to 2.5 in terms of Mw/Mn, a specific surface area (BET) of 7 to 13 m2/g and a primary particle size of 150 to 250 nm. This micropowder has flowability in a melt state and is used as a modifier. Thus, it is the same type of a material as the above-described waxes (low molecular weight PTFE).
Modified PTFE fine powders are mostly processed in the form of a wire coating or a tube, and used in fields where particularly high quality is required, for example, aircraft, automobiles, chemical industries, etc. In particular, tubes are used as flexible piping in hydraulic control systems, fuel supply pipes, high pressure steam pipes, chemical liquid transfer hoses, etc. In these applications, liquid leakage caused by the breakage of tubes should be avoided from the viewpoint of safety. Thus, it is desired to improve the mechanical properties, in particular, pressure resistance of the tubes.
One object of the present invention is to provide a modified PTFE fine powder, which has good paste exrudability and transparency, and can provide molded articles having good mechanical properties, in particular, excellent pressure resistance.
Another object of the present invention is to provide a method for the production of such a PTFE fine powder.
To achieve the above objects, the present invention provides a modified PTFE fine powder comprising agglomerates of colloidal particles with an average particle size of from 0.05 to 0.5 xcexcm which comprise a copolymer of tetrafluoroethylene and at least one olefin monomer selected from the group consisting of a fluoroalkyl vinyl ether of the general formula:
X(CF2)nOCFxe2x95x90CF2xe2x80x83xe2x80x83(I)
wherein X is a hydrogen atom, a fluorine atom or a chlorine atom, and n is an integer of 1 to 6, and a fluoroalkyl vinyl ether of the general formula:
C3F7(OCF2CF2CF2)m[OCF(CF3)CF2]pOCFxe2x95x90CF2xe2x80x83xe2x80x83(II)
wherein m and p are independently an integer of 0 to 4, provided that the sum of m and p is not 0 (zero), preferably C3F7OCFxe2x95x90CF2, the content of the olefin monomer being from 0.02 to 0.3 wt. %, preferably from 0.03 to 0.2 wt. %, wherein the copolymer has a molecular weight distribution Mw/Mn of from 1.5 to 4.5, preferably from 2.0 to 4.0 and a standard specific gravity of from 2.135 to 2.175, preferably from 2.140 to 2.160.